For sensing the true position of a moving part of a machine, different sorts of electrical measuring systems have been put forward in the prior art such as precision potentiometers having wipers which are mechanically joined to the moving machine part, limit switches which are actuated by the moving machine part when it gets as far as certain positions, inductive pickups etc. Such sensing systems or pickups may, generally speaking, be used with fluid-powered ram-cylinder actuators. However, all parts fixed on the outside of an actuator are likely to be damaged or to become fouled in operation. Furthermore, such sensing systems or pickups are frequently so designed that they are readily damaged and do not have a long working life under rough conditions, for which the actuator may well as such be designed.
Furthermore, special purpose position pickups for ram-cylinder actuators have been put forward in the prior art which are completely within the cylinder or are part thereof so that the shortcomings noted are not to be feared. Such position pickups have air or electrical limit switches provided in the end walls of the cylinder so as to be actuated directly by the ram, which may be in the form of a piston and piston rod, when the ram comes to an end position, such pickups, however, having the shortcoming that they may not be used for sensing other positions of the piston.